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The mode of operation of the 68HC12 can be configured using the BKGD, MODB, and MODA pins. The operating modes fall into two categories: normal operating modes and special operating modes. The special operating modes are used chiefly for testing and development. For example, in the special peripheral mode, the CPU is not active. This allows an external processor to control on-chip peripherals for testing purposes.
Both the normal and special operating modes include single-chip modes in which the external buses are not active. In these modes, ports A, B, and E can be used for general purpose I/O. In the expanded modes, external data and address buses are used to allow off-chip devices to be incorporated in an application.
The expanded modes use ports A and B for the high and low bytes, resepctively, of the 16-bit external address bus. In narrow mode, the 8-bit external data bus is multiplexed on port A. In wide mode, the 16-bit external data bus is multiplexed on ports A and B.
On our lab demo board, BKGD is always connected to 1. MODB and MODA are selected using switches 1 and 2 on the upper board. (These should be off to select single chip mode.)